The present invention is in the field of measuring/inspecting techniques and relates to a method and a system for controlling the operation of a processing toot for processing workpieces. The invention is particularly useful in the manufacturing of semiconductor devices to control the operation of photolithography tools to optimize the entire photolithography process.
The manufacture of semiconductor devices consists of several procedures applied to a semiconductor wafer to define active and passive elements. The wafer is prepared and one or n)ore layers are deposited thereon. Thereafter, the process of photolithography is performed in which the surface of a wafer with a pattern conforming to circuit elements is formed. An etching process applied to the uppermost layer follows the photolithography, By desirably repeating these processes, a multi-level semiconductor wafer is produced. Thus, photolithography is one of the main steps in the manufacture of semiconductor devices. It actually consists of the optical image transfer of a pattern form a mask to a semiconductor wafer.
It is a common goal of the semiconductor industry to minimize features on, a wafer, namely to make the pattern finer and finer. Owing to the fact that optical systems used for image transfer reach their limitations. die lithography process should meet higher requirements of its operational performance. This means finer process control, as well as the development of new lithography equipment and chemicals. The major steps of the photolithography process are as follows:
(a) coating a wafer with a photoresist material (PR);
(b) exposing the PR to UV radiation through a mask in order to produce a latent image of the mask on the PR;
(c) developing the exposed FR in order to produce the image; and
(d) measuring and inspecting the wafer.
During the exposure of PR to UV light the PR becomes more or less soluble in a developing solvent, as compared to the exposed PR, thereby producing a positive or a negative tune image, respectively.
FIG. 1 illustrates a common photolithography tools arrangement, a so-called, xe2x80x9clink arrangementxe2x80x9d, generally designated 1, for carrying out the photolithography process. The main idea underlying the implementation of such a link arrangement is that each tool is dedicated to serve the next one in the series, so as to minimize process/tool variations. The link arrangement 1 is composed of two main parts: a phototrack 2 and an exposure tool 3. The phototrack 2 is formed by a coater track 4 and a developer track 5, associated with cassette load/unload stations, designated 4a and 5a, respectively. A robot (not shown) loads the wafer from the cassette station 4a to the coater track 4, arid, when the coating procedure is complete, transfers it to the exposure tool 3. Here, the pattern on a mask is aligned with a structure already on the wafer (registration) by an optical means installed inside the exposure tool 3, and the wafer is exposed to electromagnetic radiation through the mask. After exposure, the robot transfers the wafer to the developer track 5 and then to the cassette station 5a. Additionally, several different baking procedures are implemented during the steps (a)-(C). The coater track 2, exposure tool 3 and developer track 5 eve tightly joined together in order to minimize process variability and any potential risk of contamination during photolithography which is a very sensitive process.
The measurement/inspection step is carried out with a metrology tool 7, which is typically a big, stand-alone machine, that serves for the serial critical dimensions (CD) measurement. CD metrology tool 7 measures the width of representative lines, spaces and line/space pairs on the wafer. The operation of a conventional CD metrology tool is based on two main methods: scanning electron microscope (CD SEM) and atomic forced microscope (CD AFM). CD measurements typically talk place after the developing step. To this end, xe2x80x9cdevelopedxe2x80x9d wafers are taken out of the link arrangement 1 and transferred to the separate CD station occupied by the tool 7. Data obtained during the CD measurements is analyzed with a processor 8 (which is typically integral with the CD metrology tool), and then a some sort of feedback is provided (e.g. an alarm in case of a width out of the permitted range) and transmitted to a relevant unit in the production line.
The quality of the entire photolithography process is defined by a combination of tolerances for all relevant parameters that can influence the final image transfer. The main parameter that should be controlled (and the easier to be adjusted and compensated) is the exposure dosage, i.e. the amount of energy reaching the PR.
According to one known technique, so-called xe2x80x9csend ahead waferxe2x80x9d, a pilot wafer is sent through the arrangement 1, namely through the coating-exposure-developing steps, applying a certain recommended exposure dose (and time), and then undergoes CD measurements. The results of the measurements will be the basis for set-up conditions of the entire lot, or for a correction signal to be applied to the tool 3 prior to the exposure of another wafer in the lot, i.e. a feedback loop. The whole sequence of such a xe2x80x9csend ahead waferxe2x80x9d procedure can take many hours, during which valuable time of the production tools is not fully utilized and the wafers"" flow is delayed. According to another technique, each lot is the basis for the next lot to run in this process representing a so-called xe2x80x9clot-to-lot controlxe2x80x9d. By considering the results of the previous lot, a small correction can be made. However, a certain increment in the risk exists, because the entire lot may be lost. Both of these techniques are time, labor and materials consuming and usually do not reveal any problematic root.
It is known that the photolithography provides sufficient results at certain levels of PR bleaching. Unfortunately, owing to the fluctuations of scan speed and light intensity, it is very difficult to reproduce each time the optimum exposure dose.
The most popular method used in production for providing a measurement directly correlated with the photoresist lithography image is a so-called xe2x80x9coptimal exposure testxe2x80x9d. According to this method, a wafer coated with a photoresist material is exposed through a mask using a sequence of different dosages. Following the exposure and development steps, the dose is estimated as a function of line width, utilizing the electron microscopy technique. Notwithstanding that this method considers all the relevant operations and materials of the entire photolithography process (i.e. coat, expose, develop, bakes, resist. etc.), it consumes expensive useful time of the exposure equipment.
U.S. Pat. No. 5,620,818 discloses a photolithographic dose determination technique, which utilizes diffraction of a latent image grating for constructing a calibration curve. This technique is not compatible with on-line production control, because of the following features. It requires that a special mask be designed and a special test wafer, having all the relevant stack layers, be created. A large area of a test structure is needed to provide a sufficient signal-to-noise ratio. Additionally, to consider each layer and each resist when constructing the calibration curve, a sequence of gradual exposures of the mask on the wafer should be conducted.
U.S. Pat. No. 5,635,285 discloses several methods of determining the correction for exposure. One of them is based on an exposure with a phase shift mask, which suffers from the need of an additional alignment procedure. Another method uses the known FLEX technique for exposures in several focus conditions to overcome the limits of depth of focus (DOF). This method has alignment and magnification error related problems. Yet another method is based on the use of an additional xe2x80x9cout of focus illuminationxe2x80x9d. More specifically, additional radiation is added outside the depth of focus and the mask operates as a gray scale regime. Consequently, the method is xe2x80x9cmask regime dependentxe2x80x9d, and therefore should be applied for each mask area, each layer and each product separately.
U.S. Pat. No. 4,474,864 discloses a method for dose calculation presenting an initial calculation procedure that relates to the construction of calibration curves for the first exposure set up. This calibration is implemented by the coating and gradual exposure of a few transparent wafers for measuring the absorption resulting from bleaching at a certain single wavelength. However, the method suits a laboratory measurement procedure and not a real time process control since it is time consuming, and requires a long preparation procedure. This method does not consider any simultaneous measurement of thickness and refractive index (only absorption) which may vary during exposure, thereby affecting the absorption. Moreover, this method is based on the assumption that the reflection is negligible, which may actually yield an error. According to this patent disclosure, the deduction of a calibration curve is based on a single wavelength. This indicates that the measurement values have no statistical averaging that can decrease the error of the measurement itself.
In view of the above, it is evident that existing techniques for exposure dose determination/correction cannot be used as on-line manufacturing steps, and fail to provide high accuracy and automatic analysis and xe2x80x9cfeed-forwardxe2x80x9d dose control, rather than xe2x80x9cfeed-backxe2x80x9d. The existing methods lead to the waste of wafers and other materials like photoresist and solvents, as well as the waste of costly/useful time of the photolithographic tool. Hence, they reduce the production rate (i.e. throughput) of the lithographic tools. Additionally, current methods do not allow for accurate and fast determination of the optical parameters of the PR layer, such as an absorption coefficient k and a refraction index n, and therefore do not allow for the direct dosage correction.
There is accordingly a need in the art to improve the quality of a photolithography process used in the manufacture of semiconductor devices by providing a novel measuring method and system.
It is a major object of the present invention to provide such a method and a system that can be used for controlling parameters of the photolithography tools in production.
There is provided, according to one aspect of the invention, a method for automatic optical control of at least one working parameter of a processing tool to be applied to a working area of a workpiece for providing certain process results, said at least one working parameter of the processing tool affecting at least one parameter of the workpiece under processing, wherein the processing tool has a preset value of said at least one working parameter prior to the processing of the workpiece, the method comprising the steps of:
(a) providing a measuring tool to be applied to the workpiece prior to its processing by the processing tool;
(b) applying the measuring tool to the workpiece for:
(c) measuring said at least one parameter of the workpiece and generating measured data representative thereof;
(d) analyzing said measured data with respect to said preset value of the working parameter and to said process results and determining whether said preset value should be corrected for providing said certain process results when applying the processing tool to said workpiece; and
(e) upon detecting that said preset value should be corrected, calculating a correction value and generating data representative thereof.
The main idea of the present invention consists of the following. A workpiece progressing along a production line is to be processed by a processing tool. The working parameter of the processing tool is typically tuned to a preset value. During the processing of the workpiece with the processing tool, the value of this working parameter affects some parameters of the workpiece. The processing is expected to provide certain desired values of these workpieces"" parameters (constituting the process results). However, owing to the fact the various procedures were applied to the workpiece before it arrives to the processing tool (which is usually the case considering such a workpiece as a semiconductor wafer progressing on the production line), these procedures may unpredictably influence on the parameters of the workpiece. Consequently, the preset value of the working parameter needs to be corrected so as to meet the requirements of the real before-processing conditions of the specific workpiece and to satisfy the process results. For the purpose, a novel controlling method is proposed. The method consists of measuring the workpiece""s parameters before the processing, and analyzing the same, as well as the preset value of the working parameter and the process results, to determine a correction value to be applied to the preset value for achieving the process results when applying the processing tool to die measured workpiece. This technique of measuring the operational workpiece before its processing and adjusting the processing tool parameter accordingly represents a feed forward closed loop.
Certain reference data provided and used for performing the measurements and analysis of the measured data. The reference data is representative of at least one calibration curve in the form, of the at least one parameter of the workpieces as a function of the at least one working parameter of the processing tool. The reference data also comprises an optical model based on nominal values of certain features of the workpiece for obtaining theoretical data representative of the at least one parameter of the workpiece. The optical model presents theoretical data (mathematical equation) in the form of the intensity radiation as a function of wavelength, wherein the radiation is that returned (reflected) from an illuminated area of the workpiece. The reflected radiation depends on the required parameters of the workpiece in accordance with known physical effects relevant to the specific known kind of workpieces.
If the workpiece to be processed follows a preceding (already processed) workpiece of the same group, (e.g. one or more lot or batches in the case of wafers) the calibration curves are known (already obtained). When dealing with a xe2x80x9cnewxe2x80x9d group of workpieces of the known kind, the calibration curves are prepared with respect to a first-coming workpiece in the group. In order to prepare the at least one calibration curve, a so-called xe2x80x9cset-up operations stagexe2x80x9d should be performed. This stage consists of applying a desired number of xe2x80x9ctest cyclesxe2x80x9d to the operational workpiece within a xe2x80x9ctest areaxe2x80x9d thereof. Such a workpiece as wafer is typically formed with a test area located out of the working (patterned) area and having features similar to the features of the working area.
The test cycle consists of before-process measurement, test process and after-process measurement steps, sequentially applied to the test area and being carried out by the measuring tool located as described above. To this end, the measuring tool is adapted for processing the workpiece similar to the processing tool, a ratio between the working parameters of the measuring and processing tools being of a predetermined value. The desired number of such test cycles are performed by small movements to corresponding number of test sites (portions) within the test area using different values of the working parameter of the measuring tool for each cycles and each time determining the values of the required parameters of the workpiece.
From the calibration curve a recommended value of the working parameter can be determined. It should be noted that the recommended value might be given by a manufacturer. In this case, the calibration curve serves for determining whether this given value satisfies the process results, and, upon detecting that it does not satisfy the process results, for calculating the correction value to be applied to the recommended value. Additionally, during the preparation of the calibration curves, the nominal values of some features of the workpiece could be updated and the optical model so optimized could be further used for measurements.
Each measurement is performed by illuminating at least a portion (test site) within the test area by a predetermined incident radiation spectrum and detecting radiation returned (reflected) from the illuminated area. Measured data so obtained is in the form of the radiation intensity as a function of wavelength. Using a fitting procedure between the measured and theoretical data, the required parameters can be determined and analyzed to generate data representative of the correction value. This data may be xe2x80x9cfed forwardxe2x80x9d to the processing tool to adjust the value of its working parameter for obtaining the process results for this specific measured workpiece.
Preferably, the workpieces are wafers, the production line being a conventional photolithography arrangement. The working area of the wafer is an area, which is formed or is to be formed with a desired pattern. The processing tool to be controlled is, preferably. an exposure tool, the working parameter to be connected being the exposure dose. However, in general, the processing tool may be any one of those used in the photolithography arrangement (i.e. coater, developer, etc.). The at least one measured parameter of the workpiece is the wafer""s reflectivity (i.e. reflectivity of either a substrate or a photoresist layer on the substrate), PR refraction index, absorption coefficient or thickness.
Thus, according to another aspect of the present invention there is provided a method for automatic optical control of at least one working parameter of a processing too to be applied to a working area of a workpiece for providing certain process results, wherein said processing tool is a part of a photolithography tools arrangement, said at least one working parameter of the processing tool afffects at least one parameter of the workpiece under processing, the processing tool has a preset value of said at least one working parameter prior to the processing, the method comprising the steps of:
providing a measuring tool to be applied to said workpiece prior to its processing by the processing tool;
applying the measuring tool to said wafer for:
measuring said at least one parameter of the wafer and generating measured data representative thereof;
analyzing said measured data with respect to said preset value of the working parameter and to said process results for determining whether said preset value should be corrected for providing said process results when applying the processing tool to said wafer; and
upon detecting that said preset value should be corrected, calculating a correction value and generating data representative thereof.
According to yet another aspect of the present invention, there is provided a measuring tool for an automatic optical control of at least one working parameter of a processing tool which is to be applied to a workpiece for processing a working area thereof for providing certain process results, said working parameter affecting at least one parameter of the workpiece under processing, wherein the processing tool has a preset value of said at least one working parameter prior to the processing, the tool comprising:
(1) a processing channel adapted for processing the workpiece similar to the processing of the processing tool, a ratio between the working parameter of the processing channel and processing tool being of a predetermined value;
(2) a measurement channel adapted for measuring said at least one parameter of the workpiece and generating measured data representative thereof;
(3) an actuator associated with said processing channel and said measuring channel for selectively actuating one of them; and
(4) a processor coupled to said measurement channel, the processor being responsive to said measured data for determining and analyzing said at least one parameter of the workpiece, and calculating a correction value to be applied to the working parameter of the processing tool prior to the processing of the workpiece, so as to obtain said process results when applying the processing tool to said workpiece.
According to yet another aspect of the present invention, there is provided a production line having at least one processing tool adapted for processing successive workpieces progressing along the production line so as to provide certain process results, wherein said processing tool has a at least one working parameter thereof that affects at least one parameter of the workpiece under processing, the processing tool having a preset value of said at least one working parameter prior to the processing of said workpieces, the production line comprising a measuring tool installed so as to be applied to an operational workpiece prior to the processing thereof by the processing tool, the measuring tool being adapted for measuring said at least one parameter of the workpiece and determining whether said preset value should be corrected for providing the process results when applying the processing tool to said operational workpiece.
More specifically, the present invention is used with a photolithography tools arrangement for controlling the exposure tool parameter and is therefore described below with respect to this application.